Modifiers of Age of Onset in ALS Studied Using Patient iPS-derived Motor Neurons Amyotrophic lateral sclerosis (ALS) is a uniformly fatal age-associated neurodegenerative disease that destroys motor neurons. Although all ALS patients are adults, age of onset can vary widely, with a range of about 60 years. The precise factors that determine the interaction of aging and neurodegeneration in ALS are unknown. I performed a re-analysis of published expression profiles from ALS-patient motoneurons and uncovered a group of genes expressed at levels highly correlated with age at symptom onset. These genes are potential modifiers of ALS disease severity. The overall goal of this work is to use motoneurons derived from patient-specific induced pluripotent stem cells (iPS cells) to validate these candidate disease onset- modifiers and to discover new ALS-dependent gene expression changes in iPS-derived motoneurons. Until recently, it was not possible to study living, pre-symptomatic human motoneurons from ALS patients. However, we have derived multiple iPS lines from ALS patients and controls and differentiated them into patient-specific motor neurons (iPSMNs). The first aim of this work will be to characterize and improve upon a system that I have developed to purify iPSMNs from the mixed cultures produced during differentiation. In this aim, iPS-derived motoneurons will be infected with a lentivirus that encodes a motoneuron-specific reporter. Virally labeled cells will then be characterized to reveal any motoneuron-subtype biases of the reporter strategy. Following this, I will attempt to improve upon the already substantial enrichment of motoneurons achieved through this technique by isolating cells that co-express the viral reporter and motoneuron-enriched surface markers that I have identified in previous work. [[I will then use this enrichment strategy to test the hypothesis that the candidate onset-modifying genes identified earlier will show expression patterns in iPSMNs that correlate with disease onset in ALS-patient donors, but not the age of controls. Any positive results would indicate that expression of these candidate- modifiers is a stable trait of motoneurons and may play a role in determining ALS onset. Finally, I will determine the SOD1 dependence of any validated onset-dependent expression patterns in ALS patient-derived iPSMNs. This final set of experiments will help to determine whether any validated candidate genes are true onset-modifiers or very early markers of disease. This aim has the potential to define the first ALS-related differences in iPSMNs. Understanding these differences may, in turn, lead to improved a priori measures of ALS prognosis and more relevant therapeutic targets.]] PUBLIC HEALTH RELEVANCE: This project uses stem cells generated from patients with ALS (Lou Gehrig's disease) in an attempt to understand what causes some patients to fall ill with this fatal disease early in life, while others do not show symptoms until old-age. Eventually, this research could lead to new treatments for ALS.